Plated polypropylene composition

ABSTRACT

This invention relates to certain novel articles formed from polypropylene compositions, and more particularly to articles produced by forming polypropylene compositions into a predetermined configuration and plating with a coating of one or more metals. The novel plated articles prepared from polypropylene compositions of the present invention exhibit greatly improved bonding between the metal coating and the polypropylene surface. The polypropylene composition contains a polyhydroxy aromatic compound and a resinous material. The composition may optionally contain a nucleating agent.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part of copending U.S. applicationSer. No. 108,764 filed Jan. 22, 1971, now U.S. Pat. No. 3,929,702,issued Dec. 30, 1975 which was in turn a continuation-in-part of U.S.application Ser. No. 13,042, filed Feb. 20, 1970, now abandoned.

BACKGROUND OF THE INVENTION

The present invention relates to articles formed from polypropylenecompositions suitable for metal plating. The plated articles exhibitgreatly improved bonding between the metal coating and the surface ofthe polypropylene.

Those engaged in the manufacture of metallic articles, particularlydecorative, and other non-load bearing metallic articles, have longdesired to reduce the frequently inordinate expense of manufacturingsuch articles. One of the most obvious possibilities for effecting suchreductions in costs, would be to substitute a lighter and/or lessexpensive material for a large portion of the metal in relatively heavyarticles. Another obvious possibility would be to simplify the extremelyexpensive molding, shaping and polishing procedures which must usuallybe employed in manufacturing such an article from metal. Quite some timeago, it was observed that it might be possible to obtain both of theaforementioned savings by molding or shaping the article from a plasticmaterial, and subsequently plating the surface of the molded plasticarticle with a coating of metal.

There are of course a number of criteria to be met in order to producesuch articles by molding from a plastic and subsequently plating with ametal. To name but a few, the physical property characteristics of theplastic must usually be sufficient to meet all the specificationsrequired of the final article; these physical properties must not beimpaired by degradation of the plastic due to contact with the metal;the plastic must be readily platable by vacuum deposition techniques,chemical deposition techniques, electrodeposition techniques, and/or acombination of such techniques, to produce a continuous, uniform metalcoating covering 100 percent of the area to be plated; and, the coatingof metal must adhere firmly to the surface of the molded plasticarticle.

Specific applications in which manufacturers have sought to substitute aplatable plastic composition for all or part of the former metal contentare both numerous and diverse. Recently, for example, a number ofpossible applications for platable plastic materials for the automotiveindustry have been suggested, including fabrication of grills andhubcaps. Like most other proposed applications for platable polymers,fabrication of automotive grills and hubcaps would involve the use ofsignificant quantities of materials to produce a very detaileddecorative configuration, having a plurality of geometric planes formingessentially a single surface. The use of platable polymer in suchapplications is not only advantageous from the point of view ofsubstituting a lighter or less expensive plastic for much of the metalwhich would otherwise be required, but also because of the efficiencyand effectiveness with which extremely detailed and/or structurallyshaped configurations can be achieved with plastic molding equipment.

The phenomenal increase in the commercial significance of polypropyleneis in many cases attributable to the very low specific gravity ofunfilled polypropylene. A substantial reduction in the weight of polymerrequired for a given application can often be achieved by substitutingpolypropylene for other popular thermoplastic polymers. Its low specificgravity makes polypropylene a particularly desirable polymer forfabrication of large metal plated plastic articles, such as thosedescribed above. Unfortunately, unfilled polypropylene compositionsheretofore commercially available have had certain inherent propertycharacteristics which seriously limited their use in manufacturingcertain types of articles, including automotive applications such asthose noted above. In particular, it is well known that in the absenceof large amounts of filler, polypropylene compositions are generallysubject to volumetric change with temperature. Shrinkage of the polymercomposition during cooling of the mold, particularly in areas wherethere was a change of cross-section in the molded article, resulted inso-called "sink marks," or other surface imperfections and deformities.While it is often possible to eliminate these problems by incorporatingfrom about 20 to about 30 part of filler (per 100 parts of resin) intothe polypropylene composition, this may destroy the specific gravityadvantage which may originally have made polypropylene desirable.

The general procedure for manufacturing shaped articles by metal platingan article which has been pre-formed from a polymeric material,comprises the steps of:

A. forming the polymeric material into the desired shape;

B. cleaning and treating the surface of the formed article to render itmore receptive to the metal coating;

C. depositing a metal coating by chemical or vacuum depositiontechniques onto the surface of the article; and, if desired,subsequently

D. electroplating the coated article to produce the desired deposit ofmetal coating.

It will of course be understood that each of the steps of the foregoinggeneral procedure can involve a series of specific sub-steps, which arein any given instance a function of the polymer to be employed, thespecific metal to be chemically or vacuum deposited onto the surface ofthe plastic to render it conductive, and the specific metal or metals tobe electroplated onto the surface after it has been rendered conductive.

The cleaning or surface treatment steps generally involve washing orimmersion in a de-greasing and cleaning solution to remove both surfacedirt and any traces of polymer additives which may have migrated to thesurface, particularly mold release agents or the like. Following thecleaning step the articles are usually immersed in an acid etchingsolution, and/or the surface of the shaped plastic article is otherwisetreated to improve adhesion of the subsequently deposited metal coating.

In an effort to develop polymeric compositions for plating applications,a number of thermoplastic and thermosetting polymers have beenextensively tested, particularly during the past ten years. For exampleBelgian Patent 613,430 (Feb. 28, 1962) mentions metal plating of a widevariety of polymers, including, polyolefin polymers, vinyl polymers,polyamides, polyurethanes, polystyrenes, polyacrylates, and the like.Various other types of polymers, particularlyacrylonitrile-butadiene-styrene (ABS), have also been the subject ofextensive experimentation, and even some limited commercial use.

The interest in developing commercial platable plastic compositions,and/or techniques, have also led to the investigation of a wide range ofspecial additives and procedures. For example, U.S. Pat. No. 3,466,232,to Peter S. Francis et al., discloses and claims a method of platingplastic materials in which improved adhesion is obtained byincorporating into the plastic material an additive which will be etchedout during the surface preparation step, thereby producing a largenumber of small pockets having openings in the surface which are smallerin diameter than the largest diameter through their respective pockets.

In effect, the Francis et al. procedure provides a series of undercutpores which will be filled with metal during the plating steps, therebyserving as anchoring points for the metal coating. While the proceduredisclosed by Francis et al. does provide a method of improvingadhesions, the concentration of additives used in the examples to obtainoptimum bondings, was in the order of 33 parts per hundred of resin, alevel which can have a serious deleterious effect on the basic physicalproperties of the polymer.

In the area of special additives, South African Patent Applications,Serial No. 68/2279 and Ser. No. 68/2282 were published under the title"Electroplatable Polyolefins." Both of these applications claim priorityof United States applications filed May 3, 1967, (which issued Mar. 10,1970 as U.S. Pat. No. 3,499,881) and both relate to polyolefincompositions exhibiting improved adhesion to plated metal coatings.

Application Ser. No. 68/2279 discloses polyolefin compositionscontaining from 0.25 to 1.5 parts by weight, per hundred parts by weightof resin (hereinafter "phr" for convenience) of a compatible sulfurcompound, such as dilauryl thiodipropionate, and from 10 to 60 phr of asulfate of a metal of Group II of the Period Table of Elements, such asbarium sulfate. Suitable polymers, according to the specification, arepolyethylene, polypropylene, polybutene-1, poly(4-methylpentene-1),poly(3-methylbutene-1), ethylene propylene copolymers, ethylene butene-1copolymers, and the like.

Application Serial No. 68/2282 discloses polyolefin compositionscontaining from 0.1 to 2.0 phr of a compatible surfactant (highly polarand ionic surfactants being specifically excluded), and in the preferredcompositions, from 0.25 to 1.5 phr of a compatible sulfur compound suchas dilauryl thiodipropionate. Suitable polyolefin polymers according tothis specification include, in addition to those disclosed inapplication Ser. No. 68/2279, vinyl aromatic polymers and predominantlyolefinic copolymers of hydrocarbon monomers with copolymerizable polarmonomers such as acrylic monomers and vinyl esters.

The compositions of the above noted South African patent application, aswell as the novel procedure of Francis et al., generally provideimproved adhesions in metal plated plastic articles. In fact, for sometime now certain limited commercial success has been reported inproducing metal plated articles from ABS compositions, and more recentlyfrom polypropylene. In spite of this limited commercial success however,and even with the recent improvements described above, a generallysatisfactory commercial polymer composition and complimentary procedure,have not heretofore been found.

Other types of investigations have also been made in the area ofplatable plastic compositions, investigating the effect on adhesion ofsuch things as, the types and amounts of filler variations in moldingequipment used in forming the plastic, and/or variations in theoperating conditions or a given piece of equipment, e.g., fastercooling, slower cooling, higher or lower temperatures, and the like. Ofparticular interest is the recent work of Fitchmum et al., "SurfaceMorphology in Semi-Crystalline Polymers," Polymer Letters, Vol. 7, pp.301-305 (1969). The authors indicate that the surface against which thepolymer is molded, e.g., aluminum, copper, Mylar, Teflon, etc.,apparently has an effect on the surface characteristics of the moldedpart, which in turn appears to affect the adhesion of the metal coatingwhich is subsequently plated onto the surface. Fitchmun et al. theorizethat the surface crystallinity is altered and that the surfacecrystallinity may therefore affect adhesion.

The theory of Fitchmun et al, that the changes in adhesion were directlyrelated to an alteration of surface crystallinity, will undoubtedly bethe subject of further work by both the original investigators andothers. At this time, based on the results we have observed in ourtesting, we are not in a position to form an opinion one way or theother, with respect to the surface crystallinity theory. Nevertheless,the work of Tichumun et al. would seem to have clearly established thatadhesion is, to at least some extent, a function of the surface of themolded plastic article. As will be seen in more detail hereinafter, ourtests confirm the observations of Fitchmun, that there is a directrelationship between the surface of the molded article, and the adhesionto that surface of a subsequently plated metal coating.

The evaluation of the adhesion between the metal coating and a moldedplastic substrate, is generally based on the pounds of mechanical forcerequired to peel the metal coating from the substrate, in a 90° peeltest with a tensometer. In general, the testing procedure is referred toas the "Jacquet Test," see for example, P. A. Jacquet, Transactions ofthe Electrochemical Society, Vol. 66, p. 393 (1934). A number ofmodifications or variations of the Jacquet Test have also been used, andseveral have been published, including Saubestre et al., "The Adhesionof Electrodeposits to Plastics," presented at the AmericanElectroplaters Soceity 52nd Annual Convention, July 13, 1965; and also"Testing of Plating on Plastics," Product Finishing, Vol. 18, No. 5,May, 1965. Nevertheless, the essential nature of the test remains thesame, the plated sample is prepared, and a cut is made through the metalto form one or more strips of a given width, a portion of the test stripis peeled part way back, the plated test sample piece is clampedhorizontally in the fixed jaw of the tensometer, and the peeled portionof the metal strip is clamped vertically in the pulling jaw of thetensometer. The pulling jaw is then retracted at a constant rate ofseparation and the tensile load, acting at approximately 90°, ismeasured.

SUMMARY OF THE INVENTION

The novel plated articles of the present invention are formed from apolypropylene composition comprising a polypropylene resin havingincorporated therein from about 0.1 to about 10.0 phr of a polyhydroxyaromatic compound, and from about 0.25 to about 20.0 phr of a rosinousmaterial. The preferred polypropylene compositions also contain fromabout 0.01 to about 5.0 phr of a nucleating agent.

In addition to the additives just described above (hereinafter forconvenience referred to as "the additive system of the presentinvention,") the polypropylene compositions may also containstabilizers, antioxidants, fillers, pigments, processing aids and otheradditives well known to those skilled in the art. Also for convenience,as used hereinafter, the terms "novel composition," or "composition ofthe present invention" shall be understood to be the polypropylenecomposition described above.

In the preparing, processing, testing, and evaluating of the novelarticles of the present invention, we have found a number of totallyunanticipated advantages and/or properties, with respect to thepolypropylene composition from which they are formed, the unplatedarticles formed from the polypropylene composition, and also the finalmetal plated articles. Some of the more important of these unexpectedadvantages and/or properties include:

1. the polypropylene compositions of the present invention can bemolded, extruded, or otherwise formed like any conventionalpolypropylene composition; however, after forming they appear to havecertain unique surface characteristics which in some unexplained mannerapparently facilitate more efficient and effective metal plating of theformed article;

2. in plating articles formed from the polypropylene composition of thepresent invention it is possible to obtain a much greater degree ofadhesion between the metal coating and the plastic surface than washeretofore possible;

3. even more surprising, is the fact that the improved adhesions can beobtained with modified plating or preplating procedures in which severalpreliminary steps in the plating procedure, heretofore consideredabsolutely essential in plating articles formed from polypropylene, canbe omitted;

4. a more uniform metal coating over 100 percent of the surface to beplated can be more easily and consistently obtained employing a widevariety of chemical deposition procedures;

5. the additive system of the present invention is apparently effectiveonly in the preparation of platable polypropylene compositions, andprovides little, if any, improvement in the preparation of platableplastic compositions from other polymers, even other poly-α-olefins;

6. the polypropylene compositions of the present invention are for somereason less subject to volumetric change with temperature, and thereforeso-called sink marks, and other surface imperfections and deformitiesheretofore associated with thermal contraction of unfilled polypropylenecan in most cases be so substantially reduced, as to be for allpractical purposes eliminated; 7. since the polypropylene compositionsof the present invention do not require large amounts of filler toimprove adhesion and/or molding characteristics, the chemical andphysical properties of the plated article formed therefrom areconsiderably less likely to be disadvantageously modified. Also, thelower specific gravity of the unfilled material will of coursesubstantially reduce the weight of material required to produce a givenarticle, and in almost every case, thereby effect a correspondingreduction in cost.

The foregoing is by no means a comprehensive list of the advantages orimprovements which are associated with the article and/or compositionsof our invention, but merely illustrative of what we consider the mostobviously unexpected, and/or important, and/or significant, at thistime. We have observed a wide variety of other advantages andimprovements in conjunction with the compositions of our invention, someof which will be specifically noted in conjunction with the detailedexamples and tests described below, and others which will be obviousfrom the detailed description which follows hereinafter.

The rosinous additive of the present invention should be understood toinclude, but is not limited to, one or more members selected from thegroup consisting of natural rosins, synthetic resins, and rosinderivatives. As examples of rosinous materials within the scope of thepresent invention, mention may be made of wood rosin, wood rosin esters,modified wood rosin, polymeric rosinous materials, hydrogenated woodrosins, and synthetic materials such as ester resins, polyterpenes,rosin derivatives, and the like.

A number of polyhydroxy aromatic compounds have been found to be usefulas additives in the preparation of the compositions of the presentinvention. Certain bisphenol and dihydroxybenzene type compounds,particularly thiobisphenols, sulfonylbisphenols, resorcinol andgamma-resorcylic acids have been found to be particularly useful.

Particular stabilizers, pigments, and other additives were present inmost of the test compositions described hereinafter, in order to assurea valid basis for comparison in the evaluation of the present invention.However, a wide variety of substitutes for such stabilizers, pigments,and other additives were also tested, and were found to have little, ifany, discernible effect on the improvement in the degree of adhesionobtained. Thus, while insignificantly different adhesions might beobtained with one type of stabilizer or pigment, etc., in comparisonswith other polyolefin compositions containing different stabilizers,pigments, etc., compositions containing the special additive system ofthe present invention consistently showed the same marked improvement inadhesion.

As noted earlier, the polypropylene compositions of the presentinvention may be formed into the desired shape or configuration by anyof the number of means well known to those skilled in the art, such asvarious types of conventional molding procedures, extrusion procedures,or the like, including forming into cast or oriented film, directextrusion or other types of fiber forming, and the like.

After forming, the metal plating can likewise be accomplished by anynumber of procedures well known to those skilled in the art. Forexample, there are a wide variety of specific procedures for vacuumdeposition of a thin surface coating of metal over a plastic, and aneven wider variety of specific procedures for chemical deposition ofsuch a coating. Also, following the vacuum or chemical deposition steps,the desired thickness of the metal coating and/or the coating ofadditional metals can be obtained by a number of well knownelectroplating, or other techniques.

We cannot at this time definitely define the mechanism by which thepolyolefin compositions of the present invention provide better adhesionto the plated metallic coating, however these improved adhesionproperties are observed under all of the various conditions under whichwe have conducted our tests. While we will in no way limit ourselves toany one theory by which the improved adhesion observed in testing ourcompositions might be explained, it would appear that the additivesystem of our invention causes an alteration of the physical and/orchemical state of the surface of the molded plastic, and this in turnmay cause the subsequent improvement in adhesion.

Thus, as compared with unmodified polyolefin compositions, and also ascompared with commercially available competitive "platable gradepolypropylene resins," the compositions of the present inventionconsistently exhibited superior adhesion under all types of forming,processing, and/or plating procedures. As will be seen in more detailhereafter, these differences in adhesion would appear to be improvementsin kind, rather than degree of adhesion.

As noted, the interest in developing a commercial platable plasticsystem has involved not only the investigation of polymer compositionsand additives, but also plating techniques and procedures. A number ofso-called "commercial plastic plating processes" have been developed,and some of these are well known in the art. These would include, forexample, the so-called Enthone System, and/or the so-called MacDermidSystem. Typically, however, wide variations in plating both additivesand the concentrations of such additives, as well as other significantdifferences exist within any single given system. For example, most ofthese so-called commercial plating systems, were originally developedfor plating ABS compositions, and the number of modifications, such aschanges in the concentrations in the components of the acid etchingbaths, etc., must be made where these systems are employed in theplating of polypropylene.

By way of illustration, and not by way of limitation, as examples ofplating procedures employed in preparing test samples for evaluation ofthe compositions of the present invention, mention may be made of thefollowing known general procedures:

THE ENTHONE PROCESS

1. The test sample, formed from polypropylene, or some other polymer, bymolding in a conventional press or the like, is immersed in aconditioner consisting of 40% sulfuric acid (96% concentration), 39.5%phosphoric acid (85% concentration), 3% chromium trioxide, and 17.5%water, to which has been added (per liter of conditioner) 36 g. of anadditive containing 64% of chromium trioxide and 36% sodium hydrogensulfate, for a period of 10 minutes at 85° C;

2. Then in a stannous chloride sensitizer solution containing (per literof solution) 10 g. of SnCl₂ and 40 mil. of HCl, at room temperature, for1 to 3 minutes;

3. Then in an activator solution containing (per gallon of solution) 1g. of palladium chloride and 10 ml. of HCl, for a period of 1 to 2minutes at room temperature;

4. Then in an electroless copper plating solution containing per literof solution, 29 g. of copper sulfate, 140 g. of Rochelle Salt, 40 g. ofsodium hydroxide and 166 g. of formaldehyde (37% solution) at atemperature of 70° C. for a period sufficient to obtain a continuouscoating capable of conducting electricity;

5. Then electroplating, to produce the desired deposit of metal, intests, usually a deposit of 2 mil of copper.

THE MACDERMID PROCESS

1. The molded sample was immersed in a water emulsion containing 8% byvolume terpentine plus a polyoxyethylenephenylether wetting agent, forfrom 5 to 10 minutes at 150°-160° F.;

2. Then immersed in a chromic acid cleaning solution containing 350 g.per liter of chromic acid for 1 to 2 minutes at 135° F.;

3. Then immersed in an acid etch solution composed of 190 g. of chromicacid in 500 g. of sulfuric acid, for 5 to 6 minutes at 175°-180° F.;

4. Then immersed in a solution containing 1 to 2% of analkylarylsulfonate wetting agent in water, for 1 to 3 minutes at 75°-85°F.;

5. Then immersed in an activator bath composed of a colloidal solutionof 150 g. of stannous chloride and 2 g. of palladium chloride, in 20%hydrochloric acid, for 3 to 5 minutes at 85°-90° F.;

6. Then immersed in a catalyst bath composed of hydrochloric acidcontaining 0.2 g. per liter of palladium chloride, for 1 to 3 minutes at115°-125° F.;

7. Then immersed in an electroless nickel bath for 5 to 8 minutes at85°-90° F. The electroless nickel bath contained (per liter) 10 g. ofnickel ions, 30 g. of sodium hypophosphite, 30 g. of citric acid, andsufficient ammonium hydroxide to adjust the solution to a pH of 8 to 9;

8. The samples were then placed in a conventional electroplating bath,to produce the desired deposit of metal, again in the case of testsamples, usually a deposit of about 2 mil of copper.

It will of course be obvious that the steps of the two platingprocedures described above are the substantive or functional steps, andin most cases there was at least one water rinse step between each ofthe substantive or functional steps described. Several other platingprocedures, in addition to the two procedures described in detail above,were also employed in plating sample compositions according to thepresent invention. In every case, the compositions of the presentinvention exhibited substantially the same degree of superior adhesion,in comparison with other polypropylene and/or other plasticcompositions, simultaneously plated according to the same procedure.

In order to achieve a greater degree of uniformity, it is of coursedesirable to use a single plating procedure to prepare all samples,other than those to be used in tests conducted to evaluate the effect,if any, of alternative plating procedures. Therefore, unless otherwisespecifically noted, the test samples of the present invention wereplated, using the so-called MacDermid System, described above.

Variations in testing results with a single given composition werefrequently observed, due to day-to-day changes in the various cleaningand treatment solutions, and more probably because of day-to-dayvariations in the plating baths. These day-to-day variations areparticularly common where the procedures are carried out in solutionsand plating baths of the size employed in laboratory experimentation.For this reason, the only valid comparisons which can be made, are onsamples which were processed simultaneously in each of the various stepsfrom compounding through testing.

An even more important source of variations in the results observed intesting samples prepared at different times, is the metal coatingitself. Since the adhesion is measured by the peel strength in a 90°tensometer test, the consistency of test results are a function of theconsistency of the ductility of the metal coating. In preparing sampleto test adhesion, it is therefore generally accepted procedure todeposit copper in the electroplating step, in order to achieve a moreductile metal coating. If the thickness of metal coating on one sampleis greater than that on a sample prepared in an earlier or laterexperiment, the thicker metal coating will of course be less ductile,and a comparison of peel test strengths as to such samples would beclearly meaningless.

In evaluating the results of the various detailed experiments and testsset forth hereinafter, it should therefore be noted that all of the datacontained within a particular table of comparison were obtained fromsamples which were, insofar as possible, prepared at the same time,processed at the same time, and tested at the same time.

Except as used in the specific examples hereinafter, and unlessotherwise clearly indicated by the context, as used herein, the term"polypropylene" shall be understood to include propylene homopolymers,and copolymers of propylene with up to 20 mole percent of a comonomerselected from the group consisting of α-olefins, other than propylene,having from 2 to 5 carbon atoms, and vinyl monomers. Again unless thecontrary shall be clearly obvious from the context, as used herein theterms "shaped articles," "predetermined shaped articles," "forming intopredetermined shapes," "molding," "forming," and the like, should beunderstood in the broad sense, as including any method of formingpolypropylene compositions into film, fiber, or molded articles, andshould not be construed in the limiting sense, or as restricting thepresent invention to articles of a predetermined shape formed only byso-called conventional molding techniques. The use of conventionallymolded polypropylene articles in most of the testing is also primarily amatter ofconvenience,040000000000000000000000000000000000000000000000000000000000000000

